Following perforation of a wellbore casing, a visual inspection is used to determine whether each perforation is open or blocked, and whether oil, gas or water is being produced therefrom. The results of this inspection are logged and any non-productive perforations or sources of undesirable leakage along the wellbore may be subsequently sealed. This is typically done by isolating the zone containing the perforation to be sealed, and pumping a Portland cement slurry to the zone under pressure to force the slurry into the perforation. This is known as squeeze cementing. Equipment is then removed from the isolated zone and the cement is allowed to set across the diameter of the casing. In order to regain access to the wellbore below the now-sealed perforation, the cement seal across the casing is drilled out, leaving only the plugged perforation.
One challenge in remedial cementing jobs is to provide a cement slurry that will not harden prior to placement downhole but, once appropriately placed, will quickly harden to a suitable strength. Accordingly, a retardant or accelerant is often added to the cement slurry to enable positioning of the slurry in the wellbore while avoiding premature setting of the cement. Typically, application of remedial cement to a wellbore requires one day of work, curing requires an additional day of lost productivity, and drilling out and pressure testing results in another lost day.
Unfortunately, it is common for such remedial cementing operations to fail pressure tests, requiring another remedial cementing run, following by setting, redrilling, and retesting. This seal, test, repeat process often results in many days of lost productivity in addition to the direct costs of the additional cementing procedures. New technologies to prevent or limit this source of revenue loss would be desirable.
Various cement compositions have been described for use in a variety of applications. Typical hydraulic cement slurries have a setting time that is dependent on ambient temperature, making setting during cold conditions unreliable. Moreover, hydraulic cements have low acid resistance and high porosity, which impacts their reliable use downhole in certain circumstances.
Ceramic cement formulations have been described in the art, including the chemically bonded phosphate ceramic binders and Ceramicrete® formulations of Argonne Laboratories. This cement is currently being used in various applications, such as dentistry, encapsulation of nuclear waste, runway repair, and construction projects.
For example, U.S. Pat. No. 6,518,212 to Wagh, et al., describes the formation of a chemically bonded phosphosilicate ceramic by mixing a powdered binder (composed of an alkali metal phosphate and an oxide) with water to form a ceramic cement slurry for use in various applications.
U.S. Pat. No. 6,910,537 to Brown, et al., describes a chemically bonded phosphate ceramic cement sealant for plugging boreholes. In this patent, ceramic cement slurry is formed at surface and lowered downhole within a canister for application to the appropriate wellbore location.
Further, US Published Patent Application No. 2006/0048682 to Wagh, et al., describes the use of phosphate ceramic cement formulations in oil or geothermal wells. Formulations for use in shallow and deep wells are described, and alterations to increase and decrease thickening time are discussed. Although a properly formulated composition is said to bond well to steel and to downhole formations (rocks, etc.), the need for customized adjustments to the formulation, for example to avoid premature setting of the cement based on well depth, suggests that use of these compositions to plug wellbores may be burdensome to the operator.